The present invention relates to chip resistors, and in particular, chip resistors which are sulfuration resistant.
Terminal electrodes in a majority of thick-film chip resistors and in some thin-film resistors are made of silver-based cermets. Metallic silver has several advantageous properties, including high electrical conductivity and excellent immunity to oxidizing when silver based cermets are fired in the air. Unfortunately metallic silver also has its shortcomings. Once such shortcoming is metallic silver's remarkable susceptibility to sulfur and sulfur compounds. At that, silver forms non-conductive silver sulfide resulting in open circuit in the silver-based resistor terminals. The described failure mechanism is called sulfuration phenomenon or sulfuration.
A prior art non sulfur proof thick-film chip resistor is presented in FIG. 2. It consists of an isolative substrate 1, upper silver-based terminal electrodes 2, bottom silver-based electrodes 3, a resistive element 4, an optional protective layer 5, an external protective layer 6, plated nickel layer 7, and a plated finishing layer (commonly tin) 8. Each upper electrode 2 is covered by abutting layers: (a) external protective coating 6 (glass or polymer), and (b) plated nickel 7 and finishing 8 layers. The problem is that non-metal coating 6 from one side, and plated metal layers 6, 7 from another side have a poor adhesion to each other. It promotes a small gap between them and results in ambient air penetration to the surface of silver electrodes 2. If the ambient air includes sulfur compounds, the silver electrodes will be destructed after a time. That is why commodity chip resistors often fail in automotive and industrial applications.
Two known ways to prevent the sulfuration phenomenon are used. One method involves replacing or cladding of silver by another noble metal that is sulfur proof (gold, silver-palladium alloy, etc.). A second method is to prevent the silver-based terminals from contact with ambient air (sealing of the terminals).
The disadvantages of the first method include the expensiveness of sulfur proof noble metals, the lower electrical conductivity of sulfur proof noble metals relative to metallic silver, as well as the possible incompatibility of non-silver terminals with thick-film resistor inks that are designed for use with silver termination.
The second method according to prior art (see for example U.S. Pat. No. 7,098,768, herein incorporated by reference in its entirety) consists of adding of two layers: auxiliary upper electrodes 9 (FIG. 3) and uppermost overcoat 6′. Auxiliary upper electrodes 9 cover completely each of upper silver-based terminal electrodes 2 and overlap partially the external protective coating 6. The uppermost overcoat 6′ covers the middle portion of the resistor and overlaps auxiliary upper electrodes 9.
In such a configuration, the auxiliary upper electrodes should be both platable (conductive) and sulfur proof. Examples of such material include polymer-based thick-film inks with carbon filler or base metal filler and sintering-type thick-film inks with base metal filler. The disadvantages of using auxiliary upper electrodes include low electrical conductivity and poor platability of polymer-based materials with carbon or base metal filler, possible resistance shift when sintering type inks are used for auxiliary upper electrodes, problematic implementation in small size resistors (1 mm length and less) where it is difficult to keep positional relationship between multiple layers that overlap each other in the terminal, and increased resistor thickness.
What is needed is an improved chip resistor which is sulfuration resistant.